Until the second half of the nineties, when the LEP collider started to be upgraded to investigate higher centre-of-mass energies of electron-positron collisions than those until then produced at the Z mass, the Higgs boson was not the main focus of experiments exploring the high-energy frontier. The reason is that the expected cross section of that particle was prohibitively small for the comparatively low luminosities provided by the facilities available at the time. Of course, one could still look for anomalously high-rate production of final states possessing the characteristics of a Higgs boson decay; but those searches had a limited appeal.

An initial assessment of the Run 2 chances of a Higgs boson discovery with CDF was provided by Steve Kuhlmann, who in 1995 studied how the Higgs signal could be evidenced by searching for its production in association with a W boson. The Higgs boson can be produced also alone in proton-antiproton collisions, but it is then quite hard to spot it because most of the time its decay yields just a pair of b-quark jets. Such rather featureless dijet events cannot be collected by the trigger system with nearly high enough efficiency: jet triggers are "prescaled" so that only a small fraction of jet events are sent to the output stream.

On the contrary, the associated production of a Higgs particle with a vector boson (a W or a Z), while occurring with a ten times smaller rate, offers a much better chance for a sensitive search at the Tevatron, as the W or Z decay to energetic electrons or muons provides for a perfect, efficient triggering signature. Kuhlmann had considered how the Higgs decay to pairs of b-quark jets could be reconstructed, and had started to study in detail how the corrections of the measured jet energy could improve the dijet mass resolution, boosting the sensitivity to the signal. His work evidenced that a discovery was possible, although it required large integrated luminosity - a long Run 2.

Kuhlmann's study turned out to be crucial for the future of CDF as soon as it became clear that the case needed to be made for an extended high-luminosity run of the Tevatron after the end of Run 1 and the upgrade of the accelerator. In September 1993 the Superconducting Super Collider had been canceled by a vote of the US congress. That was the machine which promised to discover the Higgs boson and explain the enigma of electroweak symmetry breaking: its demise was a very hard blow to particle physics and it left a clear void in the US HEP program. Despite that, the laboratory director John Peoples did not hide his lack of interest in the high-energy frontier; he was more interested in astro-particle developments, as he believed that directing all the laboratory efforts toward a very strong investment in Run 2 of the Tevatron was a threat to the long-term perspectives of Fermilab: it was a very visible and important project, but it did not integrate well with other research efforts that the lab had the need to pursuit.

Peoples' plan was to have a Run 2 with an upgraded accelerator complex, featuring a "main injector" capable of boosting the machine luminosity, and deliver to the experiments just 500 inverse picobarns of data -a mere five times more than what the Tevatron ended up delivering to the experiments by the end of 1995; and then decommission CDF and DZERO, and move on with other projects. Many in CDF thought that it was just crazy: the Tevatron was the highest-energy collider in the world and it would remain such for another decade. Taking 500 inverse picobarns and then disposing of it appeared a gigantic blunder. However, changing the directors' plans entailed making a very strong case for an extended running; the experimentalists had to join forces, as an effort from just one of the two experiments had few chances to succeed.

Dante Amidei was among those who started to think deep about the issue. On an Autumn day in 1994 he chanced to meet his DZERO colleague Chip Brock at the O'Hare airport. They soon found out that they shared the worry for the future of hadron collider physics in the US, and they ended up spending the time before their flight discussing how to put together an effort to increase the scope of Run 2, and how to make a strong physics case.

At first it looked as if the top quark should be the main motivation for an extended study: that particle was on the verge of being discovered, and the discovery would bring media attention and leverage to the experiments. The top quark was very heavy, and this fact could by itself be used as a hint that new discoveries were hidden in its phenomenology: maybe its decays would differ from predictions, or processes involving the new-found particle could become a tag of new physics. Supersymmetry, for instance, could in some scenarios provide an "inverse hierarchy" to its squarks, such that the superpartner of the top quark would be the lightest one. Given the natural expectation that quarks and squarks of the same generation would be coupled more tightly to each other than across different generations, this made the study of the top quark a potential way to detect supersymmetry.

Soon a grass-roots movement took shape in CDF and DZERO, and a project called "TeV 2000" was started by a meeting held at Michigan university on October 21st 1994, which was attended by over 100 Fermilab physicists. The case for a long Run 2 seemed hard to defend with top physics alone; accordingly, studies of supersymmetry, electroweak physics, and exotica were started within independent working groups. It was then that Amidei stumbled on Kuhlmann's studies, which were perfect for the scope of the report that was being produced. Kuhlmann had estimated the chances of a Higgs discovery as a function of Higgs mass and available integrated luminosity. In CDF-note 3342, titled "Will we find the Higgs in Run 2?," he wrote:

Discovering a 100-130 GeV Higgs in this mode alone, with only 5-10 inverse femtobarns, will be [...] difficult [...]. We believe an observation of a 120 GeV Higgs, as an example, requires 25 inverse femtobarns of data [...]

Today Kuhlmann's original 1995 assessment appears surprisingly accurate, given that the 125 GeV Higgs produced a little less than 3-sigma effect in the 10 inverse femtobarns of data collected by the Tevatron until 2011: multiply those 10/fb of data by 2.5 times, and you are in the right ballpark for getting a 5-sigma signal!

Regardless of the accuracy of his assessments, the study became the pivot point to steer the laboratory's plans toward a stronger commitment: it soon became clear that the way to go was to push on the Higgs discovery chances. During all reviews of the TeV 2000 proposal to extend the scope of Run 2, the audience would immediately resonate as soon as the Fermilab physicists mentioned that there was a luminosity threshold above which the experiments would have a clean shot at discovering the Higgs boson. If the Higgs was lighter than 100 GeV the required luminosity was not prohibitively high, but still much larger than the 500 inverse picobarns originally envisioned.

Comments

This is an interesting inside look at just how Fermi lab operates. I was merely trying to understand algebra and trigonometry when these things happened. Right now it seems that similar moves are being made to give Fermilab a new long term lease on life as the home of a Muon collier. Without which, as it was said in a p5 meeting I attended Fermilab should cease to be a national laboratory and just be a extension of one of our larger universities. (such as U of Chi, like a second Argonne.)

Short term neutrino physics is the main focus. R&D on a muon collider is the long term plan. If that collider does not come to fruition, I see subdivisions being built on FNAL's land. Without the possibility of some sort of collider FNAL is redundant.

You might be right, but what you call long range, I call very remote in the future range...
In parallel to the neutrino factory there are more urgent goals for the future, like
the improvement of superconducting cavities and key participation in ILC.
Let's not dream too much ahead..